Mineral particles such as calcium carbonate, kaolin, and titanium dioxide are widely used as white pigments in paper, plastics, and paints. Titanium dioxide (TiO.sub.2) pigments are the most expensive of the white mineral pigments. TiO.sub.2 pigments are used in preference to other white pigments because of their outstanding properties for providing whiteness and opacity. In plastics, only low levels (viz. 1-2%) of TiO.sub.2 pigments are needed, but this adds significantly to the cost of manufacture. To reduce TiO.sub.2 pigment cost, there has been an extensive search for additives or extenders of TiO.sub.2 in plastics.
TiO.sub.2 pigments are also used in papermaking. U.S. Pat. No. 3,458,395 describes the preparation of coated titanium dioxide pigments using a coating of aminated polysaccharide. The coated titanium dioxide is described as providing improved retention in paper making.
Calcined clay (kaolin clay) has been used as an extender of TiO.sub.2 in papermaking, and in paints. For example, the Engelhard Corporation in a publication entitled "ANSILEX 93 in Paperfilling Applications" recommends combining 16 pounds of ANSILEX 93 (calcined clay) with 48.5 pounds of TiO.sub.2 to obtain comparable brightness and opacity to the use of 57 pounds of TiO.sub.2 in a paper furnish.
Particulate calcium carbonate has been proposed as an extender for TiO.sub.2 as a whitening and brightening agent in the manufacture of paper: Chang and Scott, "An investigation into the mechanism of titaniumdioxide extension by calcium carbonate", Tappi Journal, October, 1988, pages 128-131. This study concluded that although calcium carbonate particles can enhance the light-scattering behavior of TiO.sub.2, there is no specific interaction between the particles of CaCO.sub.3 and TiO.sub.2.
It has also been proposed to pretreat a mineral filler (e.g., calcium carbonate) with an organic polymeric material (e.g., an acrylate polymer) to improve performance of the CaCO.sub.3 particles. As described in British Patent 1,405,751, a slurry is formed from the mineral particles and an acrylate polymer emulsion, and the slurry is spray dried to produce a powdery product. This patent describes ordinary spray drying. It is also known to spray dry a slurry injected with a gas to produce porous or fluffy spray-dried products. (See, for example, U.S. Pat. Nos. 3,222,193 and 4,180,593.)
In the manufacture of plastics both rutile and anastase titanium dioxide are used as whitening pigments. They can be used alone or in combination with colorants. For maximum effectiveness, rutile titanium dioxide is preferred because of its high refractive index (2.7), but anastase TiO.sub.2 is cheaper. Titanium dioxide particles are typically irregular particles with their largest dimension in a size range of 0.2 to 0.3 microns. The level of opacity of a plastic part is determined by the amount of added material that has a refractive index different than that of the substrate. Consequently, if more opacity is needed in a particular application, more titanium dioxide must be used to accomplish this where a white or light pastel color is desired. This adds to the manufacturing cost.
The requirements for a whitening agent in plastics are different than in paper making. Between paper and plastics are sufficient differentials between the refractive indices of the medium and inorganic fillers which determine light reflection effects. In papermaking titanium dioxide pigments used alone or with fillers such as kaolin clay can provide brightness, whiteness, opacity, and light scattering to the paper. Titanium dioxide extenders in paper making such as kaolin or calcium carbonate can help deagglomerate titanium dioxide particles by filling in the spaces between the TiO.sub.2 particles. No specific interaction or particle bonding has been observed.
Unlike paper, however, the refractive indices of plastic polymers are relatively high and are close or equal to those of most inorganic fillers such as kaolin. Consequently, with kaolin particles, for example, there is little or no light reflected and no benefit optically to their use as a plastic filler. When used as extenders for titanium dioxide, inorganic fillers have heretofore been limited to a few percent substitutions for the TiO.sub.2. Higher loading has resulted in losses in reflectivity, opacity, and brightness. Kaolin clay heretofore has not been a significantly effective extender for TiO.sub.2 in plastics.
For a mineral filler to function as an extender for titanium dioxide, it must possess a combination of properties which have proven very difficult to achieve. As an initial requirement, the extender must be substantially less expensive than the TiO.sub.2 pigments, while having a high refractive index and being usable in the plastic without adverse effects. When combined with the TiO.sub.2 the resulting color and opacity should be substantially as good as where an equivalent amount of TiO.sub.2 is used alone. As far as is known, up to the present invention no inorganic filler has been developed which satisfies the commercial requirements for effectively extending TiO.sub.2 in plastics.